1. Field of the Invention
The present invention is directed in general to wireless communication systems. In one aspect, the present invention relates to a method and system for controlling the common mode feedback phase margin of a variable-output-bandwidth mixer in a wireless communication device.
2. Related Art
Communication systems are known to support wireless and wire-lined communications between wireless and/or wire-lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth (BT), advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS) and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device (such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc.) communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over the tuned channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switched telephone network, via the Internet, and/or via some other wide area network.
Wireless communication devices typically communicate with one another using a radio transceiver (i.e., receiver and transmitter) that may be incorporated in, or coupled to, the wireless communication device. The transmitter typically includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna. In direct conversion transmitters/receivers, conversion directly between baseband signals and RF signals is performed. The receiver is typically coupled to an antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies them. The intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
Wireless communication devices for multimedia audio and video applications generally require a higher data rate than is currently available with devices operating under the 802.11(a), (b), and (g) standards. Several draft standards (such as the 802.11(n) draft standard) have been proposed to implement faster data rates for wireless devices. Wireless devices operating under these standards will be capable of providing data rates up to 200 Mbps. Since these standards have not been finalized, they will be referred to generically as 802.11(x) herein.
Active common mode feedback (CMFB) is a very effective way to set the DC level of a down conversion module employing an adaptive mixer used in a wireless communication device. In general, most design criteria for wireless transceivers require 45 degrees of phase margin in the output of the adaptive mixer. In down conversion modules wherein resistance loads and bandwidth-compensating capacitors are used to control the gain and bandwidth of an output signal, however, it is very common to have a phase margin for the output signal which falls below the desired design criteria. There is a need, therefore, for an improved down conversion module that employs common mode feedback to control the DC level of the output signal.